In a CMOS image sensor, which is used in a digital single-lens reflex camera and a video camera, photodiodes that perform photoelectric conversion are two-dimensionally arranged on a flat semiconductor substrate.
When an image of an object on a plane is captured through a lens, an image is formed on a curved surface on the image plane side. The curved surface is concave relative to the lens. When this image is received by a flat image sensor, aberration called “curvature of field” occurs, and a difference is generated between the focal position in a central portion of the image sensor and the focal position in a peripheral portion of the image sensor. As a result, the image quality becomes not uniform in a central portion of the image and in a peripheral portion of the image, which leads to degradation in the image quality.
In order to improve such optical characteristics, Japanese Patent Laid-Open No. 2014-116380 discloses that it is possible to achieve the effect of correcting the curvature of field by forming the image sensor so as to be curved along a concaved surface. Also, Japanese Patent Laid-Open No. 2014-116380 discloses methods for manufacturing an image sensor having such a curved shape, namely a method by which the curved shape is formed by applying a magnetic field to a substrate made of magnetic material, and a method by which the curved shape is formed by generating tensile stress in a light-receiving portion.
Meanwhile, it is known that if tensile stress is generated in a semiconductor substrate in order to form the above-described curved shape, the energy bandgap of silicon expands, and dark current decreases. Within the imaging plane, due to such characteristics, variation in dark current occurs between a region where tensile stress is generated so as to form the curved shape and a region where tensile stress is not generated. Also, even if tensile stress is generated in the whole image plane, variation in dark current occurs within the imaging plane in a similar manner if there are differences in the magnitude of stress among regions. Variation in dark current within the imaging plane can be a cause of degradation in the image quality in the cases of long exposure shooting and shooting under a high-temperature environment, for example.